This invention relates to materials which are boardband absorbers of electromagnetic (EM) radiation.
The fabrication of broadband, inhomogeneous layer absorbers has traditionally been an extremely difficult task because of the need to vary the dielectric properties of the layer from those of air to those of the lossy material. This need arises because an analysis of the Fresnel coefficient for reflected irradiance at the interface between air and a lossy material shows that the incident EM radiation will be strongly reflected unless the sharp discontinuity in electric and/or magnetic properties at the interface is smoothed out, i.e., the impedance or refractive index of the lossy material is matched to that of air ("Radar Cross Section Handbook," Volume 2, edited by G. T. Ruck, Plenum, New York, 1970, pp. 611-630).
In addition to the inhomogeneous layer absorber, a number of other absorbers have been developed to minimize the reflection of EM radiation at an air-material interface. The most important absorbers designs for flat plates are listed below ("Radar Cross Section Handbook," Volume 2, edited by G. T. Ruck, Plenum, New York, 1970, pp. 611-630.):
Single layer and multilayer Salisbury screen,
Magnetic Salisbury screen,
Single and multiple homogeneous layers (Dallenbach absorber,
Single layer and multilayer circuit analog absorbers,
.mu.=.epsilon. absorber,
Geometric transition absorber,
Low-density absorber.
All of these absorbers, including the inhomogeneous layer absorber, have been utilized in the reduction of reflection in the microwave spectral region. However, not all of the above absorbers are used for IR and visible radiation. The principal reason is that the short wavelengths of IR and visible radiation dictate that the thickness of the various layers in these absorbers are on the micrometer and sub-micrometer scale. Thin-film deposition techniques must generally be employed to fabricate IR and visible radiation absorbers, and as a result, the single or multiple homogeneous layer design is preferred.
Short descriptions of the above absorbers designs are given below, along with their disadvantages.